To protect electronic telecommunications equipment from damage due to lightning strikes or other overvoltage causing hazards, surge protectors are commonly coupled between transmission lines and ground. Surge protectors may also be used in the protection of other electrical or electronic equipment. These surge protectors offer a normally electronically open condition between the lines and ground. A voltage surge, however, causes a spark to be initiated across a spark gap. A gas in the gap becomes ionized to render the space across the gap conductive until the overvoltage causing energy is dissipated.
A typical gas tube protector structure includes a dielectric envelope for mounting two electrically spaced electrodes opposite one another. One type of prior art protector uses a relatively wide gap between faces of the two electrodes. An auxiliary electrode is used to promote an initiation of an ionizing spark. After the gas becomes ionized the resistance across the gap between the faces of the electrodes drops, and the electric discharge occurs between the faces of the electrodes. To insure a low resistance across the gap while the gas is in an ionized state, the gas pressure within the envelope is maintained below atmospheric pressure, typically at about one-tenth of an atmosphere. A disadvantage of such low pressure protectors is that any leakage of air into the envelope causes the protectors to fail in an open condition, meaning that the spark is no longer sustainable across the gap of the protector at a sufficiently low voltage to dissipate the overvoltage energy.
Various ways have been thought of to ensure that protectors fail in an electrically shorted condition. Even though a shorted condition tends to shut down the electronic equipment temporarily, such a temporary shutdown of the electronic equipment is usually preferred over the alternative, an open failure of the surge protector, which leaves the equipment without protection from damage due to overvoltage conditions.
One way to minimize or eliminate a tendency of the protectors to fail open is to narrow the gap between the electrodes and at the same time pressurize the envelope of the protectors with the ionizable gas to about the same as or to above atmospheric pressure. Such a pressurization tends to offset a decrease in the breakdown voltage which is otherwise experienced when the gaps are made more narrow than the gaps of low pressure surge protectors. A subsequently occuring leak in the envelope then does not significantly alter the resistance between the electrodes of the protector, e.g., the protector will not fail in an open condition.
A problem associated with narrowing the gap and concurrently pressurizing the ionizable gas to or above atmospheric pressure is that the resistivity per length of the spark gap is increased. Tolerances on the relatively small gap become extremely small to achieve device characteristics which fall into pre-established desirable ranges. For example, when the gas pressure in the envelope is increased to approximately one and one-half atmospheres, a desirable gap width between the electrodes of the protector is approximately 50.times.10.sup.-6 meters or 50 microns. However, eve a ten percent tolerance on such a dimension is too small and difficult to maintain in the assembly of surge protectors under typical present day manufacturing conditions.
One prior art surge protector uses a relatively wide gap in conjunction with a gas under partial atmospheric pressure within the envelope of the surge protector. A second narrow gap is established by a dielectric spacer external to the envelope. The second gap is intended to function as a safety gap to initiate an arc at a slightly higher than normal voltage, but only when the surge protecting function within the surge protector has failed in the open condition. The exposed location of the electrodes forming the second gap do, however, tend to alter the characteristics of the second gap.